ABSTRACT

Ground‐motion prediction equations (GMPEs) for the horizontal and vertical spectral accelerations (SAs) from the offshore area off the Sagami Bay is presented in this article. To compare the ground motions on the seafloor with those at onshore sites, an onshore GMPE was derived for the onshore stations adjacent to the offshore sites and including those in the Kanto basin sites. The offshore dataset includes 738 three‐component records, and the onshore dataset includes 3775 records; both datasets are derived from the same set of 233 earthquakes selected for this study. The local site conditions of the offshore sites are investigated by implementing the mean horizontal‐to‐vertical response spectral ratios because soil condition data are unavailable. Individual site correction terms are used for the offshore sites due to differences among the site conditions, whereas site‐class terms are used for the onshore sites based on the travel‐time‐averaged shear‐wave velocity to 30 m (VS30). A comparison between the offshore and onshore models shows that the horizontal SAs for the whole periods and vertical SAs for moderate and long periods of the offshore motions are considerably larger than those of the onshore motions due to the deep deposition layer comprising soft sediment blanketing the seafloor; the vertical SAs of the offshore motions are slightly smaller than those of the onshore motions at short spectral periods. The inconsistency of the vertical motions at short periods may be due to the combined effect of the deposition layer and water layer. However, the vertical site amplification for short periods is independent of the water depth, and the reason of high‐frequency deficiency in vertical components need further investigation.

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